2012 will be a leap year. Check date calculation and validation routines for February/March.

Beeing a leap year, it has 366 days. If you are doing statistics or are working in Microsoft's Zune team, this might be important to remember.

The Mesoamerican Long Count calendar (also known as the Mayan calendar) rolls over into a new epoch. Software that works with that (History related data sets, Museums) might have to make a few bugfixes.